More particularly, a helicopter is provided with a main rotor providing it with lift and propulsion. In order to direct the helicopter, a pilot modifies the pitch of the blades of the main rotor, i.e. their angle of incidence relative to the incident air flow.
Consequently, the rotorcraft includes a swashplate provided with a stationary lower swashplate and a rotary upper swashplate. The stationary lower swashplate is connected to pilot flight controls, generally via three distinct control lines, while the rotary upper swashplate is connected to each of the blades via a respective rod.
The swashplate is thus a controlling swashplate that slides vertically along the mast of the main rotor while oscillating in all directions about a ball joint.
The oscillations and the vertical travel of the swashplate, as controlled by the pilot, give rise to the variation in the pitch of the blades that enable the pilot to direct the helicopter.
Conventionally, the pilot controls the swashplate via mechanical controls that are connected to the swashplate by rods. Nevertheless, the forces the pilot needs to exert in order to move the swashplate are very large.
Consequently, a servo-control is then arranged on each of the control lines. The pilot then acts on the servo-controls without making any particular effort, and the servo-controls then relay the order from the pilot to  the swashplate and consequently adjust the pitch of the blades in the required manner.
Similarly, a helicopter is provided with a tail rotor and the pitch of its blades can be adjustable via a servo-control.
Naturally, the same applies to airplane ailerons, for example, when operated via servo-controls.
It should be observed that certain modern aircraft have electrical controls that replace the mechanical connections connecting the flight controls to the servo-controls.
In usual manner, servo-controls comprise at least an outer cylinder surrounding a piston, with the movement of the piston relative to the cylinder being controlled by a hydraulic distributor actuated by the flight controls of the helicopter pilot.
Two embodiments then coexist.
In a first embodiment, the piston is secured to a fixed point of the helicopter with the cylinder moving to act on the swashplate. The person skilled in the art refers to this type of servo-control as a “moving cylinder servo-control”,
In contrast, in a second embodiment, the cylinder is secured to a fixed point of the helicopter with the piston then moving to act on the swashplate. The person skilled in the art thus refers to this type of servo-control as a “moving piston servo-control”.
In addition, regardless of the embodiment, servo-controls can be of the kinds referred to as “single cylinder” or as “two-cylinder”.
A single cylinder servo-control then has a cylinder defining a retraction chamber and extension chamber separated by a piston. The retraction chamber and the extension chamber are then fed from a single hydraulic valve.
Such a servo-control performs its function well. Nevertheless, for safety reasons, the person skilled in  the art tends to use a two-cylinder servo-control when forces exceed a certain level.
A two-cylinder servo-control then has a bottom cylinder and a top cylinder both surrounding a respective piston. In each of the cylinders, the piston defines a retraction chamber and an extension chamber.
In addition, two distinct hydraulic distributors actuated by a common input lever connected to the pilot controls serves to feed the retraction and extension chambers in the bottom and top cylinders, respectively.
It will readily be understood that the person skilled in the art requires complete reliability from the servo-controls, which is to be expected, since any malfunction of a servo-control is generally considered as a failure that is catastrophic from the safety point of view.
Cylinder redundancy thus serves to address the problem of safety. Nevertheless, it is also essential for each of the hydraulic valves that actuates a respective servo-control cylinder, to be completely safe itself.
Document FR 2 460 435 discloses a first hydraulic distributor having rotary distributor members.
The hydraulic distributor has a cylindrical main distributor member referred to as a “core”, that is mounted to turn in a stationary sleeve.
In addition, an emergency distributor member is arranged between the sleeve and the main distributor member.
The sleeve has bores for feeding fluid under pressure to the retraction and extension chambers in such a manner as to cause the cylinder to move.
In addition, the core is provided with passages having progressive openings that are arranged in its periphery so as to allow hydraulic fluid to flow from one bore of the sleeve to another when the core is turned by a manual control. 
Likewise, the emergency distributor member is pierced by radial openings to allow fluid to pass from one bore of the sleeve to another if the emergency distributor member is turned.
In normal operation, the main distributor member is not suitable for causing the emergency distributor member to turn. Nevertheless, in the event of the main distributor member seizing, it then entrains the emergency distributor member by friction, thereby allowing fluid to flow from one bore in the sleeve to another.
Consequently, the hydraulic distributor operates correctly even in the event of seizing.
Nevertheless, in such a situation, it is appropriate to warn the pilot of the aircraft that the servo-control is not operating properly. The servo-control is therefore fitted with a device for detecting seizing.
On turning, the emergency distributor member sets three balls into axial movement, which move a lever via a plate. The lever moves a rod that actuates a contactor to trigger an alarm. For questions of sealing, the rod is provided with a dynamic sealing gasket to keep the contactor separate from the hydraulic system of the hydraulic distributor, with the gasket being said to be dynamic insofar as it provides sealing relative to a moving part.
That detector device is effective. Nevertheless, the presence in particular of the dynamic gasket is problematic since it is difficult and expensive to obtain good long-term reliability from such a dynamic seal.
The dynamic gasket can then sometimes lead to harmful hydraulic leakage, requiring the servo-control to be dismantled for repair. Such an operation is expensive, in particular in terms of maintenance hours and of aircraft unavailability.
A second servo-control is also known that has a second hydraulic distributor provided with main and  emergency distributor members that are substantially cylindrical, and that move not in rotation but axially, referred to respectively as the “main slide” and as the “emergency slide”.
The main distributor member then moves along its own axis of symmetry to allow hydraulic fluid to pass from one bore of the sleeve to another.
In the event of seizing, the main distributor member moves along the emergency valve member along said axis of symmetry, thereby allowing the hydraulic fluid to pass.
That second hydraulic distributor is fitted with a device for detecting seizing.
A rod perpendicular to the said axis of symmetry is secured at one end to the emergency distributor member. That first rod is provided with a dynamic seal keeping the device for detecting seizing separate from the hydraulic system of the second hydraulic distributor.
Under the effect of the emergency valve member moving in translation, the rod causes a plurality of levers to pivot, thus actuating an electrical contactor to trigger an alarm.
As above, the second hydraulic valve with slides gives satisfaction but the problems associated with hydraulic leakage via the dynamic gasket remain.